Electromagnetic pickups are employed to convert the vibration of the plucked strings in an electric guitar to sound. Such pickups comprise generally a permanent magnet system, including one or more pole pieces, to establish a magnetic field within which the strings vibrate, and a coil structure disposed in the field to generate electrical signals corresponding to the perturbations in the field caused by the vibrating strings. These electrical signals are amplified to drive acoustic loudspeakers to provide the musical sounds.
A variety of electromagnetic pickups have been developed to date. Typically, they include a permanent magnet in magnetic circuit with a number of pole pieces equal to the number of strings on the instrument. The coils for developing the electrical signals are wound on bobbins so arranged that the pole pieces are within the coils to allow the magnetic field developed by the magnet and pole pieces to envelop the coil. Each string, when set into vibration, causes variations in the magnetic field in the vicinity of its associated pole piece or pieces, which variations are converted into electrical signals by the interaction of the magnetic field with the coil.
The basic coil, pole piece and permanent magnet arrangement has taken a number of different physical forms. However, in all configurations known to the inventor, the permanent magnet element has been in direct contact with the pole pieces to assure maximum field strength of the developed magnetic field. One known arrangement, comprising two coils, each with a pair of pole pieces for each of the strings, is illustrated in U.S. Pat. No. 2,896,491, granted Jul. 28, 1959 to Lover. The dual coil arrangement serves to cancel out hum induced by interfering electromagnetic fields and thus improves sound quality. This type of device is commonly referred to as the "humbucker" pickup. An improved form of the two-coil pickup is illustrated and described in copending application Ser. No. 07/578,763, filed Sep. 6, 1990, assigned to the present assignee which issued on May 12, 1992 as U.S. Pat. No. 5,111,728. Other known forms of pickups, such as described in U.S. Pat. No. 2,911,871, granted Nov. 10, 1959, to Schultz, employ a single coil and pole piece array.
These prior art arrangements are characterized in that the permanent magnet element is in direct contact with the pole pieces to provide a strong magnetic field. It is known in the art that variation in tonal quality can be achieved by changing the strength of the magnetic field in which the strings vibrate. As will be understood, the magnetic field tends to dampen the vibrations of a string; the stronger the field, the greater the damping. Differences in the damping factor will affect qualities of the sound generated by the strings, such as the length of the note generated, referred to as the "sustain" and distortion. To the musician, reduction in the damping effect by a limited amount will produce a more pleasing musical sound. However, in the past, this has been accomplished by reducing the field strength of the permanent magnet employed in the pickup in ways that have produced undesirable side effects.
In one such expedient, magnetization of the permanent magnet material is calibrated such that the material is magnetized to less than the maximum possible field strength. Magnets so produced suffer from the tendency to lose magnetic strength relatively quickly, thus affecting sound quality and requiring replacement more frequently. Another expedient is to use an alloy having a weaker field strength, such as Alnico 2 or 3, in place of the stronger field strength and less costly, Alnico 5.